“Observations of interstellar objects are still rare enough that we learn something new on every occasion,” said Shane Byrne, principal investigator for the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter. That rarity is precisely what made early October’s encounter with comet 3I/ATLAS only the third confirmed interstellar object an extraordinary opportunity for planetary scientists.
On October 2nd, 2025, 3I/ATLAS swept within 30 million kilometres of Mars, bringing the comet within range of several spacecraft. NASA’s MRO pivoted from its usual task of imaging Martian terrain to track the comet with HiRISE, capturing visible-light frames at a scale of roughly 30 kilometres per pixel. Images show a diffuse coma approximately 1,500 kilometers across produced by sunlight-driven outgassing of dust and ice. Though far coarser than HiRISE’s typical 25-centimetre surface resolution, these are among the closest optical views any NASA mission will get of the object. The data will help constrain the nucleus size, coma particle colours, and possibly reveal sunward jets or fragments.
MRO’s targeting was a feat of precision engineering. The comet hurtled through space at about 130,000 mph, which made it the fastest object ever imaged by the orbiter. Engineers had to avoid glare from Mars’ atmosphere and background starfields bright enough to overwhelm the faint target. The last off-Mars targeting was tried in 2014 when MRO and MAVEN observed comet Siding Spring.
While MRO delivered the optical detail, NASA’s Mars Atmosphere and Volatile EvolutioN, or MAVEN, orbiter provided ultraviolet spectroscopy using its Imaging Ultraviolet Spectrograph between September 27th and October 7th. It mapped hydrogen emissions coming from three distinct sources: Mars’ own atmosphere, interplanetary hydrogen, and hydrogen released by 3I/ATLAS when water molecules broke apart in sunlight. Multi-wavelength imaging enabled the instrument to set upper limits on the comet’s hydrogen-to-deuterium ratio a key tracer of its formation environment. “The detections we are seeing are significant, and we have only scraped the surface of our analysis,” said Shannon Curry, the principal investigator for MAVEN.
These detections complement the earlier ones made from Earth-orbiting platforms. Starting in late July, the Ultraviolet/Optical Telescope on the Neil Gehrels-Swift Observatory detected OH emission at 3085 Å-an unambiguous proxy for water-at a heliocentric distance of 3.51 AU. An estimated water production rate was approximately 1.35 × 10²⁷ molecules per second (~40 kg/s), surprisingly high for that distance where water sublimation is usually very inefficient. To explain this, modeling suggests at least 20% of the surface must be active if the water comes directly from the nucleus, well above the 3-5% characteristic for solar system comets. Near-infrared spectra also provide evidence for large icy grains in the coma, which can act as extended water sources.
It has also been found, through ground-based photometry and spectroscopy, to have a rather red spectral slope of up to 38% per 1000 Å in the near-UV, redder than that of 2I/Borisov and consistent with that of D-type asteroids and outer solar system bodies. That would suggest either a dust population dominated by larger grains or significant space weathering during its interstellar journey. High-resolution multiband imaging from facilities like Palomar and Apache Point has estimated dust production rates of 0.1 to 1.0 kg/s, while ejection velocities have been estimated between 0.01 and 1 m/s for micron- to millimeterscale particles.
From the Martian surface, a unique perspective came from NASA’s Perseverance rover: on October 4, its Mastcam-Z imaged the comet as a faint point against star trails-the result of long exposures necessary to detect such a dim target without tracking motion. Though scientifically limited compared with orbital data, these images demonstrate the capability of surface assets to contribute to transient event campaigns.
3I/ATLAS has since emerged from solar conjunction showing increased brightness, new ultraviolet and X-ray activity, and more pronounced color changes. Its closest approach to Earth will occur on December 19, 2025, at about 2 AU no threat, but still offering continued observation opportunities. To researchers, the Mars flyby dataset represents a rare multi-platform, multi-wavelength snapshot of an extrasolar body in mid-transit, capturing its dust physics, volatile release, and spectral character before solar heating alters its surface further.
